This says these can be permanent extenders, how would this be done?

Yasssss! They are amazing as HH extenders! You simply cut the dread off at the weft, then you can stitch them onto your dreads or use a crochet method to attach them. Personally, I like this method: http://www.doctoredlocks.com/permanent-extenders-dreadlocks/ but a lot of other people like the crochet method: http://www.doctoredlocks.com/dollylocks-reattaching-dreads/